01 - 
                Preheat oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper.
              
              
              
                02 - 
                Mix graham cracker crumbs, melted butter, and sugar until well combined. Press the mixture firmly into the bottom of the prepared pan to form a crust.
              
              
              
                03 - 
                In a large bowl, whisk together sweetened condensed milk, sour cream, lemon juice, and lemon zest until smooth.
              
              
              
                04 - 
                Pour the filling over the crust and spread it evenly.
              
              
              
                05 - 
                Bake for 20–25 minutes, just until set. Avoid browning the top.
              
              
              
                06 - 
                Let the bars cool to room temperature, then refrigerate for at least 2 hours.
              
              
              
                07 - 
                Cut into bars and top with whipped cream or powdered sugar before serving, if desired.